The present invention concerns a Raney catalyst for hydrogenation, a process for producing it and a process for producing the sugar-alcohol using the same.
Actual sugar-alcohols produced industrially include sorbitol, mannitol, maltitol, xylitol and others and they are used in quantity as food additives, industrial materials or pharmaceutical materials.
In general, these sugar-alcohols are produced by the heating reaction of sugars under hydrogen pressure in the presence of a hydrogenation catalyst.
A supported ruthenium catalyst, a Raney catalyst or the like have been used as hydrogenation catalysts for sugar-alcohol production.
Though the supported ruthenium catalyst presents a very high catalytic activity, it has the disadvantage of sugar isomerization, decomposition and polymerization during the hydrogenation.
For solving this problem, the British Patent No. 867,689 describes a Ruxe2x80x94Pd catalyst on an activated carbon carrier wherein ruthenium is added to palladium, however, the purity of sorbitol produced by the patent remains between 93.5 and 97.5%.
On the other hand, the Japanese TOKKYO-KOKAI-KOHO (18-month Publication of Unexamined Patent Application) SHOWA (hereinafter referred to as TOKKAISHO) 51-4370,describes a Ru catalyst on a zeolite aluminosilicate carrier while the Japanese TOKKAISHO 51-82208 describes an example of glucose hydrogenation by Ru catalyst on a crystalline clay aluminosilicate, but their results are not satisfactory because the purity of sorbitol is not superior to 99% in either case.
Raney catalysts are activated by dissolving a part of metals such as aluminum, zinc and silicon by an alkali from alloys of catalytic metals such as nickel, copper and iron coupled with metals such as aluminum, zinc and silicon.
The catalyst presents in general a low catalytic activity and a high catalyst deterioration and the catalyst cost assumes disadvantageously a great part of the product price.
Moreover, as the catalyst is supplied mainly in powder form coupled with used by a batch method, it is necessary to provide a step for separating the catalyst from the reaction solution after the hydrogenation, thereby increasing the production cost.
In order to remedy this defect, a variety of Raney catalysts for fixed beds have been developed, none of them leading to a satisfactory solution.
For instance, the Japanese TOKKAISHO 50-099987 describes a method for a producing a Raney catalyst for fixed bed based on nickel, cobalt or copper precipitation type catalyst.
In the method, conventional nickel, cobalt or copper precipitation type catalyst is blended and formed with powder metal/aluminum alloy and then treated at a high temperature using steam. During this step, xcex3-Al2O3 acting as a binder, is generated, but since the formed body is destroyed by the dissolution of xcex3-Al2O3 in the step of activation by an alkali, it is not appropriate for the production of Raney catalyst for fixed bed.
The Japanese TOKKAISHO 47-27888 describes a method for producing a Raney catalyst for fixed bed by dropping melted alloy in or on a chilled solvent to form a catalyst and activating it.
It is important to increase the density of catalyst to be loaded and to regulate the solution flow in the fixed bed for efficient hydrogenation during the sugar-alcohol production. For this sake, it is preferable to limit the catalyst grain diameter to 4 mm or less but, if the grain diameter is too small, the resistance increases and the solution flow slows down in a way to provoke obstruction of broken catalyst, so the preferable grain diameter is 2 to 4 mm approximately.
However, if it is desirable to obtain an uniform granulating in the catalyst production, the grain diameter of this range will only be obtained by sacrificing the yield as in the method described in the Japanese TOKKAISHO 47-27888 wherein alloy grains are produced by dropping melted metal from an orifice. Additionally, alloy grains, which are out of this diameter range should be melted again so as to increase the cost as much.
Therefore, the object of the present invention is to obtain a Raney catalyst for fixed bed remedying various problems mentioned hereinbefore and to produce a high purity sugar-alcohol at a low cost using the same.
Now the present invention will be described in more detail.
In the present invention, the nickel and aluminum ratio of quenched lump alloy may be adopted between the range of 1:2 to 2:1, however, the ratio approximately 1:1 is preferable considering the alloy cost and the catalyst activity after the development.
Droplets of melted alloy are quenched forcefully by dropping into a water bath or by another way. Raney catalyst produced by activating lump alloy obtained by chilling through natural radiation may provide an initial activity but the catalyst breaks down according to the increase of use time and can not be used as fixed bed catalyst.
Preferably, droplets of melted alloy are so made to obtain their grain diameter between 1 and 15 mm after the quench.
After classification and activation as it is, the quenched lump alloy may be used as fixed bed catalyst, however, in order to increase the catalyst surface area, it is preferable that the quenched lump alloy is classified after breaking, activated and then used as fixed bed catalyst.
Either when the quenched lump alloy as it is or after break is classified, if the grain diameter is too small, it is difficult to compose a fixed catalyst layer and even when it is composed, reaction mixture flows more slowly and it is no more possible to produce sugar-alcohol with a high productivity. On the other hand, if grains are too large, surface area per unit catalyst weight decreases so as to reduce the reaction speed and the productivity of sugar-alcohol.
To the lump form Raney catalyst according to the invention, is possible to add molybdenum, tin or the like up to 15% of catalyst metal in order to afford it with a function of its catalytic property or it is also possible to add after the activation of the catalyst.
Aqueous solution of NaOH, KOH or other alkali metal hydroxide may be used as alkali for the catalyst development and its concentration is 1 to 20%, preferably 5 to 15%. The development temperature is 40 to 100xc2x0 C., preferably 60 to 85xc2x0 C.
The development rate of the obtained catalyst may be determined by the following formulation after measuring the elution amount of aluminum into the alkali by means of chelatometric titration or the like.
Development rate (%)=(elution amount of aluminum/amount of aluminum in the alloy)xc3x97100
The development rate of the lump form Raney catalyst according to the invention is 10 to 70%, preferably 15 to 60%.
The development rate is closely related to the catalyst life and the catalyst cost assumed in the sugar-alcohol production is determined by the catalyst life. The catalyst life varies according to alloy composition, kind of sugar-alcohol to be produced or others but it should assure a continuous operation.
For this sake, it is developed within the range of 10 to 70%. If the development rate is less than 10%, the expected initial activity can not be obtained and if it is higher than 70%, high initial activity is obtained but the catalyst life is shortened. This is because the catalyst becomes fragile and nickel peels off in fine powder.
The hydrogen used in the conduction of the process for producing the sugar-alcohol using the Raney catalyst according to the invention is not limited particularly, but higher purity is more preferable.
Sugars that may be hydrogenated in the invention include glucose, xylose, maltose, lactose, fructose, starch saccharificate, sucrose or the like. They can be used alone or in combination of more than one kinds.
Normally, these sugars are supplied to the fixed bed as aqueous solution within the concentration range of 30 to 60%. If the concentration is low, the productivity will be low and if it is high, it will be difficult to eliminate reaction heat so as to deteriorate the purity of sugar-alcohol.
Material sugar is normally supplied to the fixed bed within the range of SV=0.3 to 1.5.
Here, SV is determined according to the following formulation.
SV=(Material volume supplied into the reactor)/(Volume occupied by catalyst loaded into the reactor)
Hydrogen flow rate supplied to the fixed bed is, normally, LV=10 to 60 m/hr and preferably LV=15 to 30 m/hr.
Here, LV can be determined by the following formulation:
LV=(Substance volume supplied into the reactor per unit time)/(Sectional area of the reactor)
The reaction temperature for the conduction of the process for producing the sugar-alcohol using the Raney catalyst according to the invention varies depending on sugar-alcohols to be produced but is 110 to 150xc2x0 C. and preferably 120 to 145xc2x0 C. The hydrogen pressure is, normally, about 40 to 200 Kg/cm2 and preferably 50 to 150 Kg/cm2.
The reaction form using the lump form Raney catalyst according to the invention corresponds to the continuous reaction on the fixed bed including an upflow method wherein sugar solution is supplied from the bottom of the reactor and a downflow method wherein the same flows down from the top of the reactor and the both may be adopted equally.
The reactor shape used for this reaction may be either bath, tubular or tower type.
When the upflow method is adopted, LV of the solution flowing inside should be at least 1 m/hr and preferably 4 to 8 m/hr. LV of hydrogen is 10 to 60 m/hr and preferably 15 to 30 m/hr.
When the downflow method is adopted, as the hydrogen occupies more space in the reactor, the hydrogen flow might be less and the hydrogen LV is conducted within the range of 1 to 10 m/hr.
After being used for the hydrogenation of sugar under the hydrogen pressure, the lump form Raney catalyst according to the invention is collected, crushed into powder and reactivated to be reused as powder form Raney catalyst. Hence, when the present invention is conducted, the total catalyst cost may be reduced to a level lower than when the conventional powder Raney nickel catalyst is used.
The method for crushing into powder for the collected lump form Raney catalyst is not particularly limited if only powder Raney catalyst of the granuometry appropriate for the use can be obtained finally. In general, it is crushed into within the particle size of total pass through 40 mesh, preferably within the particle size of total pass through 300 mesh.
The development of powder Raney catalyst at the reactivation is conducted at the development rate of 90 to 97% under the usual conditions.
Any reaction conditions may be applied for the preparation of sugar-alcohol by the hydrogenation with the presence of powder Raney catalyst only if the purity of sugar-alcohol is not deteriorated, however, normally, the reaction is conducted with the sugar concentration of 30 to 60%, under the hydrogen pressure of at least 40 Kg/cm2, preferably 50 to 150 Kg/cm2 and at the temperature of 110 to 150xc2x0 C.
The Raney catalyst according to the invention is a catalyst for fixed bed permitting a continuous use with a high initial activity by quenching melted alloy.
According to the process for producing the Raney catalyst of the invention, Raney catalyst, which is fixed bed catalyst, may be obtained through a short production process, without using binder for shaping, moreover, the production of the catalyst requires no special equipment.
Additionally, used lump form Raney catalyst may be collected, crushed into powder and then reactivated to be reused.
Thus, the process for producing the sugar-alcohol using Raney catalyst according to the invention allows to produce a high purity sugar-alcohol at a low cost.